1. Field of the Invention
The present invention relates to a system and method for digital radiography, and more particularly to a system and method of digital chest radiography producing a single optimized image.
2. Discussion of the Background
Digital radiography provides the ability to manipulate a radiographic image in order to improve detection of abnormal findings. The adjustable parameters include density, contrast and edge enhancement by digital unsharp masking. The chest radiograph presents a particularly difficult challenge because of the large variation in x-ray transmission between the lungs and the mediastinum, as well as the great variety of abnormalities which may occur. These abnormalities range from relatively large xe2x80x9clow frequencyxe2x80x9d types of findings such as nodules and air space infiltrates, to small xe2x80x9chigh frequencyxe2x80x9d findings such as pneumothorax and fine interstitial lung disease. It is therefore difficult to devise a single form of processing which is ideal for all applications.
Storage phosphor computed radiography (CR) is the most widely used digital technique in general radiography. This system was developed by Fuji Photo Film Company, and is extensively discussed in the patent literature. CR imaging systems are configured to produce hard copy consisting of both a mildly processed image, which resembles a conventional radiograph, and a more heavily processed version of the same image. The two images are printed on a single piece of film, typically 10 inches by 14 inches in size.
The image processing parameters of the imaging systems include the characteristic curve, the average contrast gradient contrast, unsharp mask filtering (UMF) and the processing curve. The characteristic curve is a measure of the optical density of the image, that is, how light or dark the image is, as a function of exposure or X-ray intensity, and provides contrast gradient information, while the processing curve represents the relative degree of UMF as a function of pixel value. UMF parameters include the mask size (typically 5.50 mm) and the weighting factor. The weighting factor, which governs the intensity of UMF, varies from 0.5 in the mildly processed standard image to 5.0 in the heavily processed standard image. A more detailed discussion of unsharp mask filtering and its relation to chest radiography are provided by Pratt, xe2x80x9cDigital Image Processingxe2x80x9d, John Wiley and Sons, New York (1978), MacMahon et al, xe2x80x9cThe Effect of Digital Unsharp Masking on the Detectability of Interstitial Infiltrates and Pneumothoracesxe2x80x9d, Proc. SPIE, vol. 555, pp. 246-252 (1985), Ishida et al, xe2x80x9cHigh quality digital radiographic images: Improved detection of low-contrast objects and preliminary clinical studiesxe2x80x9d, Radiographics, vol. 3, pp. 325-328 (1983), Loo et al, and xe2x80x9cInvestigation of basic imaging properties in digital radiography: 4. Effect of unsharp masking on the detectability of simple patternsxe2x80x9d, Med. Phys. vol. 12, pp. 209-214 (1985).
In the mildly processed image, a characteristic curve as shown in FIG. 1A is typically used to retain good contrast in the lungs. The characteristic curve of FIG. 1A shows that at low pixel values, the optical density is slowly varying. As the pixel value increases, the optical density varies more rapidly. At higher pixel values, the optical density levels off. A typical chest image obtained using the characteristic curve of FIG. 1A is shown in FIG. 2A. This moderately steep contrast gradient provides good contrast in the periphery of the lung in a standard dual image CR chest format, for example, but provides poor mediastinal detail. This approximates to a poor quality conventional screen/film radiograph, such as might be achieved with a moderately high contrast film. This is clearly not acceptable for use as a single image, as information in low density areas is deficient.
The more heavily processed standard image uses a characteristic curve as shown in FIG. 1B. This more heavily processed image provides impressive detail in low density areas of the image such as the mediastinum and soft tissues. Fine details, such as septal lines and pleural fissures are also enhanced. A typical chest image obtained using the characteristic curve of FIG. 1B is shown in FIG. 2B. The enhancement is achieved by using a relatively straight characteristic curve with a low overall contrast gradient, in combination with pronounced UMF. The flat characteristic curve produces a wide latitude effect, with retention of information in both low and high density areas. This would produce an unacceptably xe2x80x9cflatxe2x80x9d appearance in the absence of UMF. The processing effectively restores local contrast and enhances high frequency detail. However, the conspicuousness of pulmonary infiltrates and other relatively large low contrast abnormalities is markedly diminished in the more heavily processed image. This is due to the low overall contrast, as well as an increase in appearance noise due to UMF. Therefore, this type of image, if used in isolation, is also unacceptable. The ideal single image would retain large area contrast in the periphery of the lungs, while enhancing local contrast in low density areas to increase visibility of mediastinal, retromediastinal and retrodiaphragmatic details.
The prior art system uses a processing curve as shown in FIG. 3. This curve shows the relationship between the intensity of the UMF applied to the image and the local pixel value of the image. FIG. 3 gives the pixel value as a function of weighting factor fraction, which is the degree of the UMF blurred component added to the processed image, with 1.0 indicating that the degree of blurred component added is the same as the original digitized image. The UMF weighting factor is altered according to local image density. In this case, the processing curve is non-linear as different intensities of UMF are applied according to the density of the image. In the very low density areas, such as may occur in the mediastinum and upper abdomen, the UMF weighting factor is 0. The weighting factor rises rapidly to 1.0 (100%) in the higher density areas such as the lungs.
FIGS. 2A and 2B considered together illustrate the standard dual image format where the mildly and heavily processed images are placed on a single film. While it is clear that these dual images are complimentary to some extent, it has not been demonstrated that this dual standard image format is optimal from either a diagnostic or operational point of view. Although the use of dual images is logical in theory, this format requires double the amount of film necessary for a given image size, and provides half the image size for a given area of film when both the mildly and heavily processed images are printed on a single film. Having two images printed on a single film produces images which are undesirably small, and this small size both impedes the effectiveness of the system and the diagnostic accuracy. The small size of the images has been found deficient by radiologists and clinicians.
It is therefore an object of the present invention to provide a novel system and method of digital radiography which overcomes the disadvantages of the prior art noted above.
It is another object of the present invention to provide a system and method of digital radiography which produces a single image which improves the visibility of detail in low density areas while maintaining good overall contrast.
These and other objects are achieved by a digital imaging system having means for obtaining a digital radiological image of a subject having at least one low density region, image processing means for processing said digital image using weighting factors, said processing means using first weighting factors in said at least one low density region of said digital image, and second weighting factors smaller than said first weighting factors for regions of said digital image other than said at least one low density region, storage means for storing said digital image and said processed digital image, and output means for producing a representation of said processed digital image. The system further includes control means for controlling the operation of the system, the control means including means for allowing an operator to select processing parameters used in said image processing means.
In the image processing means, a characteristic curve with a moderately steep gradient is used which will provide improved visibility of detail in low density regions of the digital image while minimally reducing the conspicuousness of certain infiltrates and nodules. The imaging means further uses a processing curve in conjunction with unsharp mask filtering having maximum unsharp mask filtering in the low density regions of the digital image and a constant amount of unsharp mask filtering less than said maximum unsharp mask processing in regions of the digital image other than the low density regions.
The system according to the invention therefore produces a single image having markedly improved detail in the low density region while maintaining the conspicuousness of other relatively large low contrast abnormalities.
The above objects are also achieved by a method according to the invention including the steps of obtaining a digital radiological image of a subject having at least one low density region, processing said digital image using first weighting factors in the at least one low density region and second weighting factors smaller than the first weighting factors in regions of the digital image other than the at least one low density region, and producing a representation of the processed digital image.
The processing step may include using a characteristic curve having a moderately steep contrast gradient, and a processing curve in conjunction with unsharp mask filtering wherein maximum unsharp mask processing is carried out in the at least one low density region of the digital image and a constant amount of unsharp mask filtering less than said maximum unsharp mask filtering is carried out in regions of the digital image other than the at least one low density region.